The present invention relates to a method of controlling a continuously variable transmission.
In a known method of controlling a continuously variable transmission which is disclosed in Tokkai Sho. 57-161346 which has a corresponding U.S. patent application Ser. No. 362,489 and a corresponding European patent application No. 8210250.1, a shift command signal is obtained by retrieving a memorized shift pattern with a given throttle opening degree and a given vehicle speed, and in accordance with the shift command signal, a shift motor is controlled so as to establish a desired reduction ratio. The shift motor rotates at the same speed when operated and actuates a shift control valve in such a manner as to adjust the distribution of oil pressure between cylinder chambers of drive and driven pulleys. This means that the shift motor rotates at the same speed in effecting an upshifting toward small reduction ratio upon releasing the accelerator pedal for coasting, as the shift motor does upon depressing the accelerator pedal for acceleration.
However, this known method has the following problem. That is, the revolution speed of the shift motor is set high enough to provide a good response to downshifting upon depressing the accelerator pedal for acceleration. Thus, the shift motor rotates at the same high speed upon releasing the accelerator pedal for coasting, causing a rapid upshifting. This causes a rapid drop in the engine speed and a substantial change in engine torque. This change in torgue is transmitted to a driven shaft of a transmission, producing substantial shocks upon releasing the accelerator pedal for coasting.